After reliability of a Broker, the most important issue for Day Traders, is the Commissions. I have always argued, breaking even with IB can mean a 30 to 80% NET PROFITS with another Broker say like AMP who are very reliable if, you do volume and could negotiate a good rate with them. (I also have a good size account with IB used for swings and not scalps).
My belief is if you trade more than 500rt, futures contracts/month or more, then shop around for rates and forget, IB, TS etc. etc. Just a $1 difference in comm. at 500rt per month for a $10k account equals 5% NET profits per month in additional gains!!!!!!
So, pay the most attention as in all biz, to the cost of doing biz. period.